Audio microphones are commonly used in a variety of consumer applications such as cellular telephones, digital audio recorders, personal computers and teleconferencing systems. In particular, lower-cost electret condenser microphones (ECM) are used in mass produced cost sensitive applications. An ECM microphone typically includes a film of electret material that is mounted in a small package having a sound port and electrical output terminals. The electret material is adhered to a diaphragm or makes up the diaphragm itself. Most ECM microphones also include a preamplifier that can be interfaced to an audio front-end amplifier within a target application such as a cell phone. Another type of microphone is a microelectro-mechanical Systems (MEMS) microphone, which can be implemented as a pressure sensitive diaphragm is etched directly onto an integrated circuit. The MEMS sensor is typically implemented on a separate die. In combination with an ASIC on another die, a MEMS package is assembled by putting both chips into a single acoustic package. In modern microphones, a MEMS sensor is combined with application specific circuit (ASIC) in a single acoustic package with the ASIC performing the readout of the signal delivered by the MEMS and communication with the external world in either an analog manner (so called analog silicon microphones) or by involving an analog-to-digital conversion and exchanging digital data (digital silicon microphones), e.g., with a Codec unit on a mobile device.
Environmental sound pressure levels span a very large dynamic range. For example, the threshold of human hearing is at about 0 dBSPL, conversational speech is at about 60 dBSPL, while the sound of a jet aircraft 50 m away is about 140 dBSPL. MEMS microphone may be carefully designed and constructed to withstand high intensity acoustic signals and faithfully convert these high intensity acoustic signals into an electronic signal.
Besides performance, another important factor relates to production costs. Conventional ASIC of silicon microphones may use a number of non-standard or special components that increase the unit cost. For example, high voltage components if used can increase the cost of the final product significantly A ‘standard CMOS process’ is a process including core devices for logic designs and devices for I/O handling, the latter able to handle a maximum of 3.6 V. Accordingly, standard CMOS devices include core devices and I/O devices, for example, which may be devices with thicker gate oxides.